In a wheeled vehicle powered by an internal combustion engine, when the driver releases the throttle operator, such as the throttle pedal, the throttle valve almost completely closes. As a result very little air can be supplied to the engine. When this happens, if the vehicle is in movement and the engine is still connected to the wheels, the wheels want to turn the crankshaft of the engine at a speed corresponding to the speed required to move the vehicle at the speed the vehicle is currently going. However, because of the position of the throttle valve, a vacuum is created in the engine, and the torque applied on the crankshaft by the wheels needs to work against this vacuum. As a result, the engine slows down the vehicle or, in the case of a vehicle going down a hill, at least reduces the vehicle's acceleration. This is known as engine braking.
One of the main advantages of engine braking is that, by assisting in reducing the speed of the vehicle, it can help reduce wear on the brakes normally used to brake the wheels.
Certain vehicles are provided with a continuously variable transmission (CVT) to transfer torque from the engine to the wheels. A CVT has a driving pulley, a driven pulley and a belt looped around the pulleys to transmit torque between the pulleys. In most situations for a vehicle having a CVT, releasing the throttle operator would result in engine braking.
However, some CVTs have a driving pulley that is actuated centrifugally. Centrifugally actuated driving pulleys have a pair of sheaves that move closer together as the speed of the driving pulley increases. As such, in some centrifugally actuated driving pulleys, at low driving pulley speeds the sheaves are too far apart to clamp the belt therebetween. Therefore, releasing the throttle pedal when the driving pulley speed is low would not result in engine braking since the belt turns freely relative to the driving pulley and the rotation of the wheels and the driven pulley is not opposed by the engine's resistance. This would occur for example when a vehicle starts going down a hill from rest with the engine idling.
In order to address this problem, some centrifugally actuated driving pulleys are provided with a clutch, or other mechanism to cause the belt to transfer torque to the crankshaft of the engine if the belt turns freely relative to the driving pulley. In one example, the driving pulley is provided with an overrunning clutch which causes the belt to apply torque to the crankshaft if it turns relative to the crankshaft/driving pulley by more than a certain speed. When the overrunning clutch is engaged, engine braking is applied.
Although these mechanisms provide a solution to the problem of the engine not providing engine braking under certain conditions, they also add cost, weight and complexity to the vehicle.
It would therefore be desirable to provide a solution to the problem of vehicles having a CVT with a centrifugally actuated driving pulley that do not have engine braking under certain conditions.